THE ACID OPEN-HEARTH PEOCESS.
SECTION Xa.—Nature of the charge in a steel-melting furnace. —In acid open-hearth practice the shell is first lined with nine inches or more of clay brick. The furnace is then heated.to the working temperature, and sand is spread in successive layers over the entire hearth. Each layer is heated to a full heat for about ten minutes or until it is "set," so as to be hard, the sand being selected so that it will give a dense and solid bottom. When finished, the thickness of the lining should be from 18 to 24 inches. The area of the cavity for holding the charge will be determined by the size of the furnace, for the depth of the metal should be about 12 to 15 inches in a 5-ton furnace and from 18 to 24 inches when the charge is 30 to 50 tons. If the bath is shallow, the oxidation is excessive; while. if deep, the melting is slow.
The constituents of the charge vary in different places. Sometimes pig-iron alone is used, but when scrap can be obtained it forms part of the mixture. It is necessary, however, to have a certain amount of pig-iron to. protect the iron from oxidation. The stock must be low in sulphur and phosphorus, as there is no elimination of these elements.
The content of silicon, manganese and carbon is not limited by narrow bounds, for these elements are oxidized during the process and their presence in greater or lesser amounts alters the working of the charge rather than the composition of the product. In the manufacture of soft steel it is the usual practice, when scrap is available, to regulate the proportion of pig-iron so that the bath, after melting, shall be free from silicon and manganese, and shall contain from three-fourths to one per cent, of carbon. During the elimination of this element, the metal is in continual ebullition, and its temperature and condition, as well as the character of the slag, may be controlled in preparation for recarburizing and cast-